Three approaches to data analysis were compared to describe competitive interactions between wheat and Italian ryegrass. Replacement series were performed using the two species at total densities of 100, 200, and 400 plants/ m2, and separate monoculture experiments for each species at densities from 33 to 800 plants/m2. Approaches to data analysis included: 1) conventional analysis of replacement series experiments, 2) development of synthetic no-interaction responses from monoculture experiments for comparison with results from mixtures, and 3) responses of the reciprocal yield of individual plants to variation in densities of the two species. Wheat was the superior competitor to ryegrass; however, the three approaches varied in ability to quantify this competitive relationship. The conventional replacement series analysis was least sensitive in describing the influences of either density or proportion on the plant association. The synthetic no-interaction approach provided the most detailed analysis of the influence of proportion on the species interaction. The reciprocal yield approach provided the simplest and most sensitive analysis of the joint influences of density and proportion. The latter approach also provided the most quantitative analysis of the influence of density on the species interaction. Plant density and species proportion are important variables for interpreting the process of plant competition.
Application . A quantitative understanding of the competitive influence of woody and herbaceous vegetation on young conifers is required to make appropriate vegetation management decisions in new plantations . Knowing the biological thresholds of competition for the survival and stem-volume growth of conifer seedlings, such as ponderosa pine, can provide a basis for evaluating the need and objectives for vegetation control treatments .Abstract . Patterns of survival and stem-volume growth for planted seedlings of ponderosa pine (Pinus ponderosa Doug] . ex Laws .) competing with various levels of woody and herbaceous vegetation were derived from three previous studies-one in Oregon and two in Montana. Negative hyperbolic curves of opposite concavity describe the relation between the abundance of woody or herbaceous vegetation and (1) the survival and (2) the stem volume of the pine seedlings . From these curves, two types of competition thresholds for managing forest vegetation are identified : (1) maximum-response threshold-a level of vegetation abundance where additional control measures will not yield an appreciable increase in tree performance ; and (2) minimum-response threshold-a level of vegetation abundance that must be reached before additional control measures will yield an appreciable increase in tree performance . The maximum-and minimum-response thresholds for pine stem volume occurred at lower levels of vegetation abundance than those for pine survival . Thus, forest managers may need to consider ponderosa pine survival and stemvolume growth as separate objectives when managing woody and herbaceous vegetation in young plantations . Knowledge of maximum-and minimum-response thresholds also can be used to improve herbicide prescriptions .
The relationship of predawn needle water potential of ponderosa pine (Pinusponderosa Dougl. ex Laws.) and the foliage density of neighboring plants was examined after experimental gradients of foliage were established on two sites in northwestern Montana. In late summer, soil water content linearly decreased in relation to the amount of foliage of herbs and shrubs. Predawn needle water potential of pine seedlings planted along the gradient ranged from −0.5 MPa when competitor foliage was near zero to less than −3.0 MPa when such foliage was more extensive. Even when neighboring plants had a small amount of transpiring foliage at this time of year, the pine seedlings had an increased level of water stress.
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